The present disclosure relates to fluid flow control devices and more particularly to portable infusion pumps.
The primary role of an intravenous (IV) infusion device has been traditionally viewed as a way of delivering IV fluids at a certain flow rate. In emergency situations, such as natural disasters, industrial accidents, traffic accidents, police or battlefield actions, the best outcomes for trauma victims occur when an IV can be administered to prevent shock and stabilize the patient for transport to a medical facility. In practice, the limited battery function, the large size and the complexity of modern pumps has required first responders to rely on gravity infusions or delay in starting an infusion until the patient is safely inside a secure care area. This delay in providing fluids and medicine increases the mortality risk for those needing care, and gravity infusions are inconsistent at best in transport situations, sometimes running faster, sometimes slower, and sometimes not at all depending on the relative height of the fluid source and the patient's systolic blood pressure. Clearly then, there is a need for a portable pump that can be used by people with only limited training in trauma situations. Such a pump would allow greater control of infusions that are currently delayed or set and run as gravity infusions and subject to the variability of gravity infusion during patient transport and would add a significant level of safety and benefit to trauma patients, helping to stabilize them during transport to a secure care area.
In clinical practice, it is common to have fluid delivery goals other than flow rate. For example, it may be important to deliver a certain dose over an extended period of time, even if the starting volume and the actual delivery rate are not specified. This scenario of “dose delivery” is analogous to driving an automobile a certain distance in a fixed period of time by using an odometer and a clock, without regard to a speedometer reading. The ability to perform accurate “dose delivery” would be augmented by an ability to measure the volume of liquid remaining in the infusion.
Flow control devices of all sorts have an inherent error in their accuracy. Over time, the inaccuracy of the flow rate is compounded, so that the actual fluid volume delivered is further and further from the targeted volume. If the volume of the liquid to be infused can be measured, then this volume error can be used to adjust the delivery rate, bringing the flow control progressively back to zero error. The ability to measure fluid volume then provides an integrated error signal for a closed feedback control infusion system.
In clinical practice, the starting volume of an infusion is not known precisely. This is especially true with first responders arriving in a disaster or battlefield situation. The original contained volume is not a precise amount and then various concentrations and mixtures of medications are added. The result is that the actual volume of an infusion may range, for example, from about 5% below to about 20% above the nominal infusion volume. The EMT or other user of an infusion control device is left to play a game of estimating the fluid volume, so that the device stops prior to completely emptying the container, otherwise generating an alarm for air in the infusion line or the detection of an occluded line. This process of estimating often involves multiple steps to program the “volume to be infused.” This process of programming is time consuming and presents an unwanted opportunity for programming error. Therefore, it would be desirable if the fluid flow control system could measure fluid volume accurately and automatically.
If the fluid volume can be measured then this information could be viewed as it changes over time, providing information related to fluid flow rates. After all, a flow rate is simply the measurement of volume change over time.
The formulation of the ideal gas law, PV=nRT, has been commonly used to measure gas volumes. One popular method of using the gas law theory is to measure the pressure in two chambers, one of known volume and the other of unknown volume, and then to combine the two volumes and measure the resultant pressure. This method has two drawbacks. First the chamber of known volume is a fixed size, so that the change in pressure resultant from the combination of the two chambers may be too small or too large for the measurement system in place. In other words, the resolution of this method is limited. Second, the energy efficiency of this common measurement system is low, because the potential energy of pressurized gas in the chambers is lost to the atmosphere during the testing. The present invention contemplates an improved volume measurement system and method and apparatus that overcome the aforementioned limitations and others.